In general, a dental implant is used as an artificial root composed of a metal having the shape of the crown root implanted in the jawbone where teeth are totally or partially lost so as to form artificial crowns on the artificial root after the artificial root combined with the jawbone. Such dental implants are available in varieties requiring two separate surgeries as well as varieties requiring only one.
Currently, those varieties requiring only one surgery have the aesthetic disadvantage that the metal portion of the implant is exposed above the gum line. This disadvantage does not apply in the case of those varieties requiring two surgeries, however, the inconvenience and expense of an extra surgery are disadvantages in themselves.
Dental implants can be classified according to the location of the implant such as “sub-periosteal” (beneath the periosteum), “intra-osseous” (within the connective tissue), etc. Implants can also be classified according to their shapes such as threaded implant, cylindrical implant, etc. These implants will not damage adjacent teeth, they prevent osteolysis in the gum, and they generally allow for a functionally and aesthetically adequate restoration that won't become dislodged by talking, laughing or eating. As a result of the above merits, such implants have become increasingly popular among dental patients. Such implant is disclosed at U.S. Pat. No. 5,727,943 issued to Beaty et al., entitled “SELF-TAPPING, SCREW-TYPE DENTAL IMPLANT”.
FIG. 1 is an exploded perspective view for showing the conventional implant and FIG. 2 is a projected perspective view for explaining the conditions under which an artificial tooth is placed on the implant such as is shown in FIG. 1.
Referring to FIG. 1 and FIG. 2, the implant 10 includes an artificial root 20, a connection column 30 and an artificial crown 40.
The artificial root 20 is referred to as a “fixture” and fulfills the same role as the root of a natural tooth. It is implanted into the jawbone 45 where teeth are missing. The artificial root 20 supports the whole implant including the connection column 30, which connects the artificial root 20 to the artificial crown 40.
The connection column, referred to as an “abutment”, is connected to the artificial crown 40, which is fixed in the gums above the jawbone. The upper part of the connection column 30 contains a screw hole corresponding to a screw 55 that holds the artificial crown 40 in place. On the lower portion of the connection column, the screw 55 is inserted into a locking hole 65 formed vertically through the artificial root 20. A projecting portion 70 is formed on the upper portion of the connection-column screw 55 and connected to the artificial crown 40.
In order to secure the artificial crown 40 to the artificial root 20 using the connection column 30, the implant further comprises a cone-shaped hole 75, formed in the under surface of the artificial crown 40, and threaded gold cylinder 80, extending up through the artificial crown 40. A hole 95 is formed all the way through the artificial crown 40 from above. The artificial crown 40 is mounted on the gold cylinder 80, and is fixed to the artificial root 20 by the connection column 30 using the threaded projection portion 70 and a gold screw 90 implanted through the tooth hole 95. The artificial crown 40 is secured in the mouth on the connection column 30, replacing the natural tooth.
FIG. 3 is an enlarged sectional view for illustrating the artificial root 20 implanted into the jawbone 45 in FIG. 2. FIG. 4 is an enlarged bottom view for showing the bottom of the artificial root 20 implanted into the jawbone 45 in FIG. 2.
Referring to FIG. 2 and FIG. 3, the artificial root 20 comprises an upper flange 100, a mid screw 110 and a lower portion 120.
A hexagonal projection 105 is formed on the upper flange 100 on which the connection column 30 is placed. A locking hole 65 and a stop-shoulder 125 are formed between an edge 115 of the lower end of the upper flange 100 and the mid screw 110 in order to prevent the artificial root 20 from boring excessively into the jawbone 45.
In order to facilitate the implanting of the artificial root 20 into the jawbone 45, a thread 130 is formed on the outer surface of the mid screw 110 for securing it into the insert hole 140. As shown in FIG. 4, cutting edges 135 are formed on the lower portion 120. The cutting edges 135 of the lower portion 120 partially cut the jawbone 45 with the screw of the artificial root 120; the mid screw 110 is implanted into the jawbone 45; the artificial root 20 is secured in the jawbone 45.
However, the above dental implants, currently in wide use, suffer from the disadvantage of using separate parts that are combined into a single artificial tooth.
Using the present art, it is necessary to implant the artificial root in one surgery, and then, after waiting about 3 to 6 months for the bone tissue to bond with the implanted fixture, the artificial crown must be secured to the fixture during a second surgery.
The disadvantages of this two-step process are obvious. The patient must suffer the physical, mental and financial burden of two surgeries; the patient must go three to six months without the replacement tooth; the patient must endure the pain and discomfort of two post-surgery periods of healing. Although some success has been reported with new designs that allow for connecting the abutment during the first surgery, the connection process remains cumbersome and time-consuming.
Also, there is a problem with the upper flange portion of the fixture where it comes in contact with the cortex bone. The fixture has a fine, machined surface in this area and is shaped into basically a cylindrical form. These characteristics contribute to osteolysis of the cortex bone because of the stress and shearing force that the bone is subjected to after the restoration when the patient is chewing.
The process of attaching a fixture to the jawbone using a single screw with a single thread is not easy. This technique also reduces the life of the implant because it leaves the jawbone subjected to heat. Also, it currently takes approximately 3 to 6 months for the bone to bond sufficiently with the implant to allow for the mounting of an artificial crown on the machined surface. These days, the necessary time for the bone-bonding process has been reduced with the development of various improved screw forms and surface treatments. Operations mounting an artificial crown immediately after the implanting of a fixture have succeeded in non-tooth jawbone, however, the restriction remains that the quality and quantity of remaining bone must be sufficient to facilitate this technique.
Also, implants connecting a fixture, an abutment and an artificial crown with a threaded connection suffer from slack in the structure as well as screw scraps created when the screw is tightened. This problem of slack remains unsolved, though the proper application of force when tightening the screw will minimize the problem and the development of a design that wedges the negative thread of the fixture against the positive thread of the abutment has also decreased this negative effect.
Also, in the area around the connection between fixture and abutment, bacteria can enter and multiply. This can cause inflammation around the gums. Also, the fibrous layers of the gums attach vertically and securely to the surface of a natural tooth while the fibrous layers of the gums are arranged horizontally or in a ring shape when they attach to an artificial fixture. In this latter case the bond is relatively weak and insecure.
Also, because the thread portion of the fixture uses a single thread with a pitch of about 0.6 mm on the surface of a lathe-treated cylinder, it is relatively likely to fail or “strip” in the soft bone because of a limited surface area and the resulting concentration of stress in this small area of bone. These days, double threads formed on a wedged cylinder decrease the problems encountered during operations, improve the stability of the implanted fixture, decrease the waiting time between operations, reduce the amount of heat generated by friction and spread the mechanical stress over a larger area of bone. However, the problem of limited surface area and stress distribution around the screw remain essential design flaws in the present art.
When using the related drill to bore a hole in the jawbone, generally, the shaved bone chips come out along the recesses of the cutting edge. This extraction of bone material is known to slow the bonding process. In the past, the surgical procedure took longer because all alveolus bone had to be removed and the hole had to be tapped before the fixture was implanted. However, due to recent advances, fixtures are now self-tapping, cutting down on the length of the procedure. The new problem created by this self-tapping process is that the loose trabecular bone impedes the bonding process.
Moreover, when coupling an artificial crown with a fixture immediately after the fixture is implanted, the artificial crown should be mounted on an abutment. The use of dental mucous resin, with blood shedding, makes the operation complicated and more time-consuming.